I don't think it's an accident that both of your stories had a similar ending.
The person who comes up with the original idea is probably the one with the most passion for the thing in the first place and is much more likely to put in the needed effort to see it grow.
That, and the fact that the idea in itself is not as important as the execution plan. The person who came up with it probably was also thinking about that a lot more than the person who stole it.
If it were me and I liked the idea of another person so much that I could steal it, I would probably quit my job and find a way to work with them.
The person who comes up with the original idea is probably the one with the most passion for the thing in the first place and is much more likely to put in the needed effort to see it grow.
That, and the fact that the idea in itself is not as important as the execution plan. The person who came up with it probably was also thinking about that a lot more than the person who stole it.
If it were me and I liked the idea of another person so much that I could steal it, I would probably quit my job and find a way to work with them.